changed implementation to merge-sort

sorting.cpp

 #include "sorting.h"
 using namespace std ;
+
+void merge(int arr[], int l, int m, int r) 
+{ 
+    int i, j, k; 
+    int n1 = m - l + 1; 
+    int n2 =  r - m; 
+    int L[n1], R[n2]; 
+    for (i = 0; i < n1; i++) 
+        L[i] = arr[l + i]; 
+    for (j = 0; j < n2; j++) 
+        R[j] = arr[m + 1+ j]; 
+    i = 0;  
+    j = 0; 
+    k = l;  
+    while (i < n1 && j < n2) 
+    { 
+        if (L[i] <= R[j]) 
+        { 
+            arr[k] = L[i]; 
+            i++; 
+        } 
+        else
+        { 
+            arr[k] = R[j]; 
+            j++; 
+        } 
+        k++; 
+    } 
+    while (i < n1) 
+    { 
+        arr[k] = L[i]; 
+        i++; 
+        k++; 
+    } 
+    while (j < n2) 
+    { 
+        arr[k] = R[j]; 
+        j++; 
+        k++; 
+    } 
+} 
+
+void mergeSort(int arr[], int l, int r) 
+{ 
+    if (l < r) 
+    { 
+        int m = l+(r-l)/2;  
+        mergeSort(arr, l, m); 
+        mergeSort(arr, m+1, r); 
+  
+        merge(arr, l, m, r); 
+    } 
+} 
+
 vector<int> sort_custom(vector<int> data)
 {
-	vector<int> arr(data.begin(), data.end()) ;
-	for(int i = 0; i < data.size(); i++)
-	{
-		for(int j = 0; j < data.size()-i-1; j++)
-		{
-			if (arr[j] > arr[j+1]) swap(arr[j],arr[j+1]) ;
-		}
-	}
-	return arr ;
+	//vector<int> arr(data.begin(), data.end()) ;
+    int n = data.size() ;
+    int arr[n] ;
+	for(int i = 0; i < n; i++)
+    {
+        arr[i] = data[i] ;
+    }
+    mergeSort(arr,0,n-1) ;
+    vector<int> res(arr, arr+n) ;
+    return res ;
 }
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